Prepare Mini EQ 0.7.0 release

[bhack]

Summary

• tighten preset/current-curve UX so neutral, saved, modified, imported, and deleted-preset states remain recoverable
• add settings and preset document schema validation/migration coverage
• refresh release docs, tests, and public screenshots for the 0.7.0 candidate

Validation

• .venv/bin/python -m pytest tests/test_version_metadata.py -q
• .venv/bin/python -m ruff check .
• .venv/bin/python -m ruff format --check .
• .venv/bin/python -m pytest -q
• appstreamcli validate --no-net data/io.github.bhack.mini-eq.metainfo.xml
• desktop-file-validate data/io.github.bhack.mini-eq.desktop
• .venv/bin/python tools/check_gnome_shell_extension.py --no-package
• MINI_EQ_FLATHUB_MANIFEST=../io.github.bhack.mini-eq/io.github.bhack.mini-eq.yaml tools/run_release_preflight_container.sh

Release notes

• latest published tag is still v0.6.0; this PR prepares the v0.7.0 release candidate
• container preflight passed, with expected Flathub manifest drift because the sibling Flathub checkout still points at pipewire-gobject 0.3.4 while upstream is prepared for 0.3.5
• keep draft until final maintainer release checks are accepted, including any desired Flatpak runtime/background portal smoke and Shell extension upload decision